1. Field of Invention
The present invention relates to an electroluminescent display device, a method to manufacture the same, and electronic equipment provided with this electroluminescent display device.
2. Description of Related Art
Electroluminescent (hereafter referred to as EL) display devices have been developed until now as display device for portable devices, e.g., cellular phones and PDAs, personal computers, and the like.
The EL display device is configured to include a plurality of luminescent elements, which is composed of a pair of electrodes holding an EL layer (electroluminescent layer) therebetween, in a substrate surface, and desired display is performed by independently controlling the driving of each luminescent element. This EL display device can be classified into, for example, a bottom emission type which takes light from the side of the element substrate and a top emission type which takes light from the side of the sealing element, based on the difference in direction from which the light of the luminescent layer comes. However, the bottom emission type structure has been primarily researched until now for reasons of flexibility in selection of materials and the like.
In the field of the display device, needs for upsizing, high definition, and high brightness are intensified, and intensive research on upsizing of the EL display device have been actively pursued as well. However, the upsizing of the above-described bottom emission type EL display device requires a thick wiring to supply signals to electrodes and, thereby, a problem of reduction in aperture ratio of the pixel occurs. When the aperture ratio is reduced as described above, a large current is passed through the luminescent layer in order to ensure the brightness of the pixel and, as a result, a problem of a reduction in the product life occurs as well. Consequently, in recent years, the top emission type structure in which the aperture ratio of the pixel is not affected by the structure of the wiring and the like is noted, and is intensively researched. See Japanese Unexamined Patent Application Publication No. 8-185984.
With respect to a pair of electrodes holding an EL layer there between in such a top emission type EL display device, the electrode in the sealing element side (observer side) must be a transparent electrode. In general, a metal oxide, e.g., ITO and SnO2, is used for such a transparent electrode. This metal oxide is formed by sputtering a metallic material onto a substrate in an atmosphere of a carrier gas having a constant oxygen concentration.
It is known that the light transmittance of the metal oxide varies depending on the oxygen content thereof, and at least a predetermined content of oxygen must be ensured in order to attain adequate transparency.
However, in the formation of the metal oxide by sputtering on the EL layer or an electron injection layer (or a hole injection layer) provided on the EL layer, there is a problem in that the substrate EL layer may be oxidized by active oxygen in the carrier gas and, thereby, the luminescent characteristics are degraded.